Destructive acoustic pressure oscillations, or pressure pulses, may be generated in combustors of gas turbine engines as a consequence of normal operating conditions depending on fuel-air stoichiometry, total mass flow, and other operating conditions. The current trend in gas turbine combustor design towards low emissions required to meet federal and local air pollution standards has resulted in the use of lean premixed combustion systems in which fuel and air are mixed homogeneously upstream of the flame reaction region. The fuel-air ratio or the equivalence ratio at which these combustion systems operate are much “leaner” compared to more conventional combustors in order to maintain low flame temperatures which in turn limits production of unwanted gaseous NOx emissions to acceptable levels. Although this method of achieving low emissions without the use of water or steam injection is widely used, the combustion instability associated with operation at low equivalence ratio also tends to create unacceptably high dynamic pressure oscillations in the combustor which can result in hardware damage and other operational problems. A change in the resonating frequency of undesired acoustics are also a result of the pressure oscillations. While current devices in the art aim to eliminate, prevent, or reduce dynamic pressure oscillations, the current devices fail to address both high frequency and low frequency damping devices integrated at specific locations on the inner cap, also referred to as combustor front panel.
Combustion acoustics in gas turbine engines can occur over a range of frequencies. Typical frequencies are less than 1000 Hz. However under certain conditions high acoustic amplitudes for frequencies in the 1000 to 10,000 Hz range are possible. Both low and high frequency acoustic modes can cause rapid failure of combustor hardware due to high cycle fatigue. The increase in energy release density and rapid mixing of reactants to minimize NOx emissions in advanced gas turbine combustors enhance the possibility of high frequency acoustics.
Additive manufacturing technologies can be used for making combustor inner caps, acoustic dampers, and other gas turbine structures, including technologies such as binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photo-polymerization. Specifically, metallic parts can be additively manufactured using, for instance, selective laser melting, selective electron beam melting processes, and direct metal laser melting (DMLM). In these processes, layers of metallic powder are disposed. A laser beam or electron beam is directed onto the bed of metallic powder, locally melting the powder, and the beam is subsequently advanced on the powder surface. Molten metallic substance solidifies, while the metallic powder at a neighboring location is molten. Thus, a layer of solidified metal is generated along the beam trajectory. After a processing cycle in a layer of material is finished, a new layer of metal powder is disposed on top, and a new cycle of melting and subsequently solidifying the metal is carried out. In choosing the layer thickness and the beam power appropriately, each layer of solidified material is bonded to the preceding layer. Thus, a metallic component is built along a build direction of the manufacturing process. The thickness of one layer of material is typically in a range from 10 to 100 micrometers. The process advance or build direction from one layer to a subsequent layer typically is from bottom to top in a geodetic sense.
Limitations can also apply to these methods. For instance, if an overhang structure is manufactured in one layer, the overhang structure will bend without support for any new layer of applied solidified material. As a result, a weak product quality may be found, or the manufacturing process might be canceled. While a remedy for this situation might be to manufacture support structures below overhang structures, and subsequently removing the support structures, it is obvious that an additional manufacturing step involving a removal process, in particular a cutting or chip removing process, will be required, requiring an additional process step, thus adding manufacturing time, and cost. Moreover, for certain geometries manufactured, it might not be possible or very difficult to access and remove the support structures.